1. Field of the Invention
This invention relates to an optical recording and reproducing apparatus, such as an optical disc apparatus, for effecting high density recording and reproducing and read/write compatible with media of different generations.
2. Description of the Related Art
Since optical discs not only have a high density and a large capacity but also are exchangeable like floppy discs, the optical discs have come to be noticed as storage memories to be used in the future. The exchangeability of the discs provides a merit in order to widen the fields of their uses, on one hand, but brings about a demerit in order to obtain higher density and larger capacity of the disc, on the other hand. Memories on an exchangeable disc always require read/write compatibility between a disc having a high density and a large capacity and a widely used disc. The widely used software system must be transmitted from the past to the future.
On the contrary, density has been made higher and capacity has been made larger rapidly for a fixed magnetic disc apparatus (which is usually a hard disc apparatus) on which no disc exchange is required. Rodime Corporation in England first commercialized 3.5" type hard disc apparatuses ten years ago. This apparatus is provided with two discs each having a capacity of 10 MB. The recent hard disc apparatus having the same size as this old apparatus is provided with 8 discs each having a capacity of 1 GB. The capacity has become as hundred times and the areal density has become as twenty-five times as those of the old apparatus.
The 3.5" floppy disc apparatus among the magnetic disc apparatuses has a problem about the read/write compatibility with different generations as mentioned above, and thus the capacity has been increased at an extremely slow speed. In order to ensure the read/write compatibility, the capacity of main floppy discs has been increased only By enhancing the gap lengths without changing the track widths. Usually, a floppy disc having an unformat capacity of 1 MB has a track density of 135 TPI and a linear density of 8.75 kBPI. The double capacity of 2 MB was obtained by making the linear density 17.5 kBPI and the further double capacity of 4 MB was obtained by making the linear density 35 kBPI. However, the gap length of the magnetic head having a capacity of 4 MB must be 1/4 of the gap length of the magnetic head having a capacity of 1 MB. Since, therefore, the read/write compatibility has not been able to be obtained with the conventional magnetic head of tunnel erase type, a pre-erasing method has been used.
Since the density has been increased only by enhancing the linear density in the main floppy in this way, the record density of the main floppy disc has not been able to be increased to a value which meets with that of the hard disc. It is absolutely necessary to increase a track density in order to obtain a high recording density. In doing so, a sector servo system as a tracking servo technology has been studied, and floppy disc apparatus having a large capacity using this system has been put into practice. However, these apparatuses have not been commonly used because they cannot take read/write compatibility with the conventional apparatuses. Thereafter, various attempts have been made to give the read/write compatibility to apparatuses having a high track density (each of which has one read/write gap in general). One of the methods of erasing data is such that data recorded on wide tracks at a small gap is erased by turning the floppy disc twice, for example.
It became apparent that a head having a plurality of gaps is required for the floppy discs having different gaps in order to obtain complete read/write compatibility, and floppy discs having a format capacity of the order of 20 MB have been visualized now. The apparatus equipped with these floppy discs has two gaps consisting of a gap equal to the gap of the conventional head and a narrow gap such that this apparatus can take compatibility with the conventional apparatus.
On the other hand, 12"-size additional write type optical discs for optical disc apparatuses were first put into practice for document filing, ten years ago. The recording density and the capacity have become more four times or more larger than those of the first developed optical disc apparatuses. For optical discs having a size less than 12", 5.25" size optical discs using a WO medium or an MO medium (optical magnetic medium) has been standardized as an ISO standard and commercialized. Further, a recommended standard for 3.5" size optical discs using an MO medium, which is smaller than the 5.25" size optical discs, has been proposed and these optical discs have also been commercialized. The 5.25" size optical disc has a capacity of 600 MB on both surfaces, and the 3.5" size optical disc has a capacity of 128 MB on one surface. The optical discs of both sizes have the same track density of 15.9k TPI (a pitch of 1.6K .mu.m) and the same linear density (beam density) of 16.3 kBPI (a beam length of 1.56 .mu.m).
In order to double the capacity of an optical disc apparatus, light spots must be reduced by using a light source having a short wavelength or by enhancing the NA of an objective lens. When the size of the light beams is reduced, not only the linear density but also the track density become higher. As understood from the structure of floppy discs, if the track density becomes higher, unerased portions are likely to appear on a low density disc. Thus, it becomes more difficult, in general, to take read/write compatibility, as the track density becomes higher.
For attaining compatibility on an optical disc apparatus, a method is considered for reducing the diameter of light beams emitted on the objective lens for the lower level disc so as to increase an effective .lambda./NA. In order to reduce the diameter of the light beams incident on the objective lens, it is considered that two kinds of apertures are disposed in the optical paths so as to be used alternately, or a liquid crystal aperture is placed in the related optical path. However, these methods lower the using efficiency of light. These methods, therefore, cannot be realized, now that the power of a high density light source is insufficient, and apparatuses using these methods have not yet been put into practice. There is another method using two independent heads, one being for a low density and the other one for a high density. Although the last method is the most reliable, it needs two heads (including actuators), leading to a high cost and requiring a large floor space. Further, this method has a problem that the servo system occupies a double space.
Semiconductor lasers having wavelengths of the order of 830 nm and 780 nm, respectively, and providing a power of 50 mW as recording light sources are actually used. Further, red lasers of 690 nm providing about 30 mW were started to be sold as samples. However, green and blue lasers having further shorter wavelengths have just recently been oscillated at the liquid nitrogen temperature, an thus cannot be used practically now. Since the light source used for recording requires power of about ten times more than the light used for the reading-only light source, their actual application will be delayed much later.
Attention is paid now to green - blue light sources of SHG as light sources of other short wavelengths. A semiconductor laser is oscillated as YAG or YVO at an exciting light source. Then, the wavelength of the near-infrared rays (1.06 .mu.m) is doubled by an SHG element such as KTG provided in a resonator to form a 530 nm green light source, or the wavelength of the semiconductor lasers is directly doubled to form a blue light source. The former process provides a high exchange efficiency but cannot modify the doubled wavelength. On the other hand, the latter process has a defect that the exchange efficiency is low. Therefore, it is thought that it is more and more difficult to obtain powers of the light sources usable for recording in the order of green, blue and a near-ultraviolet rays having shorter wavelengths than them.
The diameter of the beam spots can also be reduced by increasing the NA of the objective lens. The recommended value of the NA for CD is 0.45, but the present NA provided by an apparatus using an optical magnetic medium (MO medium) which can perform recording and reproduction is 0.55 which is larger than the value given by the recommended value. The larger the NA, the smaller can be made the spots in reverse proportion with the NA. When the NA is made large, it becomes difficult to manufacture objective lenses at a low cost because of the difference of lens loads. Further, the value of NA is limited by a coma due to the tilt of the disc substrate. The NA is limited to around 0.55 for a conventional plastic substrate having a thickness of 1.2 mm. The NA can be made larger than 0.55 by providing a tilt compensation mechanism or by making the substrate thin. If the thickness of the substrate is reduced from 1.2 mm to 0.6 mm, a large allowance for the tilt is permitted such that NA can be made large to about 0.65. As the substrate is made thinner further, the allowance for the tilt is made larger. However, adverse effects caused by dusts attached to and flaws formed on the substrate increase and thus the thickness of the the substrate is limited. Further, the NA cannot be made so small because the objective lens must be manufactured at a low cost. As the wavelength is made shorter and shorter, working accuracy must be made higher and higher. Therefore, the thickness of the substrate and the size of the NA are limited to substantially 0.6 mm and substantially 0.65, respectively now and in the future.
An MO medium and a phase changing recording medium (PC medium) are visualized as a rewritable recording media. A TbFeCo is chiefly used as the former media and an optical disc using this medium is standardized as an ISO Standard. However, this has problems that it cannot effect overwriting and its reproduction C/N cannot be made high because noises are chiefly shot noises of the optical detector, preamplifier noises and thermal noises due to a low reproduction signal. If green or blue light of a low wavelength were used to make the density high, the C/N of the reproduction signals would be greatly lowered and the signals could not be accurately reproduced due to the reduction of the Kerr rotational angle of the medium and the detecting sensitivity of the optical detector. In consequence, PtCo using superlattice multiplying method by which the Kerr rotational angle can be increased at a short wavelength has recently been developed.
The PC medium has been recently noted because it can perform recording and erasing between crystal and amorphous phases so as to perform overwriting. There are two kinds of the PC media, one being a GeSbTe medium for proving a reproduction signal as a negative polar signal and the other being an InSbTe medium providing a reproduction signal as a positive polar signal. The former medium effects erasing in a solid phase and the latter effects erasing in a melting phase. The erasing ratio of the latter medium is higher than the former medium because the latter medium effects erasing in a melting phase, although the former medium is better than the latter medium as long as the number of rewriting is concerned because the latter medium is exerted with a higher thermal stress than the former medium is. In this respect, the realization of the latter medium has started before that of the former medium.
The normal disc made of a PC medium comprises a substrate, a lower protecting layer, a recording layer, an upper protecting layer and a light reflection layer (heat absorbing layer) spattered on the substrate, and a UV hardened layer as another protecting layer fixed to them. Since the heat conductivity of the recording layer is low, heat generated during the recording does not spread in the recording layer but is dispersed at the light reflecting layer through the upper protecting layer disposed on the recording layer for cooling. An amorphous state in which the signals are recorded on the disc is produced in the molten portion of the recording layer, and the solid erasing (crystal erasing) takes place in the portions whose temperature is lower than that of the portion in the amorphous state. The size of the recorded mark is smaller than the light beam spot, and this effect is called a self-sharpening effect.
An optical head which has two optical sources providing light beams having different wavelengths has been developed in order to function in multiple ways. Normally, laser beam sources are used as light sources. In this optical head, one of the light sources is used for reproduction and the other light source is employed for recording and erasing. In this case, light beams having a large power is used particularly for recording and erasing. Thus, it is important that the reflected light does not return to the light sources to be incident on it as light in order that the light sources operate stably.
An example of the head provided with two light sources emitting light beams having different wavelengths is disclosed in Published Unexamined Japanese Patent Application No. 61-214146. The head has a light source for recording and reproduction and a light source for erasing, in which two polarization beam splitters compound two kinds of light beams having different wavelengths and separate the compounded light beams, such that the light beams from the optical disc travel toward the light source for recording and reproduction after their reflection. The position of the light source for erasing at which light beams are emitted is displaced horizontally from the position of the light source for recording and reproduction at which the other light beams are emitted such that the reflected light beams for erasing is prevented from returning to the light source for recording and reproduction in order to avoid adverse effects from the reflected light beams.
However, the erasing light beams pass a position out of the center of the lens system comprising an objective lens, a collimator lens and the like, and it is necessary to adjust displacement of the light emitting position.
The optical disc similar to other disc exchanging type storage memories requires a high density, a larger capacity and read/write compatibility with different generations. Since read/write is performed by circular beam spots with the optical disc, the density and the capacity cannot be enhanced by narrowing the gap along the line recording direction without changing the track width like the conventional floppy disc. With the optical disc, the density can, indeed, be made high by reducing the size of the beam spots but cannot be made low, whereby high density and large capacity cannot coexist with read/write compatibility.
A high density optical head requires recording light sources having such a high power as is required for recording. However, it is difficult to obtain inexpensive green or blue light sources now. Although two independent heads providing two different light beams spots may be considered, there may be brought about the problems on the increase of the cost and space.
With the optical disc apparatus having an optical head using two light sources of the conventional different wavelengths, light beams reflected from the optical disc does not bring about any adverse effect as returned light by displacing the light emitting position of one of the light sources from the light emitting position of the other light source, there occur the problems that the shape of the beam spots on the optical disc is deteriorated and the adjustment is cumbersome.